Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Scientific Reasons)
Scientific Reasons on Periodic Classification
1 2 6
1. Every period starts from ns and ends at ns np .
The horizontal rows of elements in the periodic table in which elements are placed in successive increasing order of
their atomic numbers are called periods. Each period corresponds to its own particular fixed quantum number (n)
value in which electron are filled in accordance with Aufbau principle accordingly the orbitals of low energy are filled
up first with electrons and only then the orbitals of higher energy are filled. Since each energy level stats with ns
1
orbital and ends at np orbital, so same sequence is followed for electronic configurations starting from lower ns orbital
2 6
upto maximum ns np .
1 2
2. Why every period starts with ns and ends at ns np6?
1-2
In the modern periodic table, s-block elements with ns configuration are placed on the left side and p-block elements
2 1-6 1
with ns np configuration are kept on the right side. Hence every period must start with ns configuration and ends at
2 6
ns np configuration.
3. Why modern classification is better classification?
Modern classification is grounded on atomic number. This classification clearly exhibit periodicity of the elements with
increasing atomic number. It also helps in assigning correct position of elements which were wrongly placed in
previous periodic table.
4. Why the elements of a group in the periodic table have same valence shell configuration?
The modern periodic table is based on atomic number. In the modern periodic table, elements are arranged in such a
way that elements with similar number of valence electrons are kept beneath each other in the same group. Thus all
the elements in the same group have same valence shell configuration resulting in their same chemical properties,
valency etc.
5. The elements of same group are chemically similar but their physical properties gradually change
down the group.
The chemical properties of elements depend upon the number and arrangement of electrons in valence shells. Atoms
with same number of valence electrons would be expected to be chemically similar.
Elements of same group have same valence shell electronic configuration, therefore, they have same chemical
properties.
The regular variation in physical properties such as atomic size, I.P., E.N., etc. down a group is called Group Trend.
Group trend in physical properties such as atomic size, ionic size, and metallic character is that they increase down
the group whereas E.N., I.P. and E.A. decrease down the group.
The physical properties of elements depend upon atomic mass and the total number of electrons in inner shells. As
going down a group, a new shell is added in each period, hence there is a regular change in physical properties.
For Example
2 1
In group IIIA elements, the number of total valence electrons are 3 (ns np ) but number of inner electrons are
different.
e.g.
B, Al and Ga have 2, 10 and 28 inner electrons respectively, due to this reason, regular change in physical properties
is observed down a group.
2 2 1
5B = 5e = 1s , 2s 2p (2 inner electrons)
2 2 6 2 1
13Al = 13e = 1s , 2s 2p , 3s 3p (10 inner electrons)
2 2 6 2 6 10 2 1
31G = 31e = 1s , 2s 2p , 3s 3p , 3d , 4s , 4p (28 inner electrons)
6. Why was Mendeleev’s periodic law modified?
Mendeleev’s periodic law was modified due to following reasons:
(i) Failure to place isotopes
(ii) Placement of alkali and coinage metals in the same group
(ii) No regular difference in atomic masses of two consecutive elements
(iv) Controversial positions of lanthanides and actinides
Conceptual Inorganic Chemistry Text Book -1-
, Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Scientific Reasons)
5 1 4 2 10 1
7. Why is electoric configuration of 24Cr is 3d 4s instead of 3d 4s while that of Cu is 3d 4s instead of
9 2
3d 4s
Both Cr and Cu shows anomalous electronic configuration in which one electron from higher energy 4s orbital jumps
to the lower energy 3d orbital in order to gain extra stability of being lower energy orbital and attaining single spin
electron (i.e. half-filled orbitals) and opposite spin electrons (fully-filled orbitals) respectively.
5 1 4 2
8. Configuration of 24Cr is 3d 4s instead of 3d 4s .
5 1 4 2
Configuration of chromium is 3d 4s instead of 3d 4s . This anomalous configuration is explained as follows:
(i) 3d and 4s sub-shells are very near to each other, hence electrons of 4s-orbital by jumping may reside in d-orbital.
(ii) Half filled orbitals are more stable (in some cases).
5 1
Thus, in Cr with 3d 4s , both 3d and 4s orbitals attain stability by being half-filled.
10 1 9 2
9. Configuration of 29Cu is 3d 4s instead of 3d 4s .
10 1 9 2
Configuration of copper is 3d 4s instead of 3d 4s . This anomalous configuration is explained as follows:
(i) 3d and 4s sub-shells are very near to each other, hence electrons of 4s-orbital by jumping may reside in d-orbital
(ii) Half filled and completely filled orbitals are more stable.
10 1 10 1
Thus, in Cu with 3d 4s , both 3d and 4s orbitals attain stability by being completely filled (3d ) and half filled (4s )
respectively.
10. Why the elements of group IA are called alkali metals?
The name alkali is derived from the Arabic word means ‘ashes’. These metals are present in the ashes of plants and
gives strong alkaline solution in water. That is why they are called alkali metals.
2Na(s) + 2H2O 2NaOH(aq) + H2(g)
Why the elements of group IIA are called alkaline earth metals?
These metals are widely distributed in the earth crust and give weak alkaline solution in water. That is why they are
called alkaline earth metals.
Ca (s) + 2H2O Ca(OH)2(aq) + H2(g)
11. Group IA elements are called alkali metals while group IIA elements are called alkaline earth metals.
Group IA elements are called Alkali Metals because they yield strong alkalis when their oxides, hydrides or hydroxides
are dissolved in water which are completely soluble in water.
2Na(s) + 2H2O 2NaOH(aq) + H2(g)
Na2O(s) + H2O 2NaOH(aq) +
Group IIA elements are called Alkaline Earth Metals because they themselves and their compounds (CaCO 3) are
found in abundance in the earth crust and yield weak bases when their oxides, hydrides and hydroxides are dissolved
in water which are sparingly soluble in water.
Ca (s) + 2H2O Ca(OH)2(aq) + H2(g)
CaO(s) + H2O Ca(OH)2(aq) +
Conceptual Inorganic Chemistry Text Book -2-
Scientific Reasons on Periodic Classification
1 2 6
1. Every period starts from ns and ends at ns np .
The horizontal rows of elements in the periodic table in which elements are placed in successive increasing order of
their atomic numbers are called periods. Each period corresponds to its own particular fixed quantum number (n)
value in which electron are filled in accordance with Aufbau principle accordingly the orbitals of low energy are filled
up first with electrons and only then the orbitals of higher energy are filled. Since each energy level stats with ns
1
orbital and ends at np orbital, so same sequence is followed for electronic configurations starting from lower ns orbital
2 6
upto maximum ns np .
1 2
2. Why every period starts with ns and ends at ns np6?
1-2
In the modern periodic table, s-block elements with ns configuration are placed on the left side and p-block elements
2 1-6 1
with ns np configuration are kept on the right side. Hence every period must start with ns configuration and ends at
2 6
ns np configuration.
3. Why modern classification is better classification?
Modern classification is grounded on atomic number. This classification clearly exhibit periodicity of the elements with
increasing atomic number. It also helps in assigning correct position of elements which were wrongly placed in
previous periodic table.
4. Why the elements of a group in the periodic table have same valence shell configuration?
The modern periodic table is based on atomic number. In the modern periodic table, elements are arranged in such a
way that elements with similar number of valence electrons are kept beneath each other in the same group. Thus all
the elements in the same group have same valence shell configuration resulting in their same chemical properties,
valency etc.
5. The elements of same group are chemically similar but their physical properties gradually change
down the group.
The chemical properties of elements depend upon the number and arrangement of electrons in valence shells. Atoms
with same number of valence electrons would be expected to be chemically similar.
Elements of same group have same valence shell electronic configuration, therefore, they have same chemical
properties.
The regular variation in physical properties such as atomic size, I.P., E.N., etc. down a group is called Group Trend.
Group trend in physical properties such as atomic size, ionic size, and metallic character is that they increase down
the group whereas E.N., I.P. and E.A. decrease down the group.
The physical properties of elements depend upon atomic mass and the total number of electrons in inner shells. As
going down a group, a new shell is added in each period, hence there is a regular change in physical properties.
For Example
2 1
In group IIIA elements, the number of total valence electrons are 3 (ns np ) but number of inner electrons are
different.
e.g.
B, Al and Ga have 2, 10 and 28 inner electrons respectively, due to this reason, regular change in physical properties
is observed down a group.
2 2 1
5B = 5e = 1s , 2s 2p (2 inner electrons)
2 2 6 2 1
13Al = 13e = 1s , 2s 2p , 3s 3p (10 inner electrons)
2 2 6 2 6 10 2 1
31G = 31e = 1s , 2s 2p , 3s 3p , 3d , 4s , 4p (28 inner electrons)
6. Why was Mendeleev’s periodic law modified?
Mendeleev’s periodic law was modified due to following reasons:
(i) Failure to place isotopes
(ii) Placement of alkali and coinage metals in the same group
(ii) No regular difference in atomic masses of two consecutive elements
(iv) Controversial positions of lanthanides and actinides
Conceptual Inorganic Chemistry Text Book -1-
, Dr Inam Ul Haq Jazbi Unit # 1 … Periodic Table (Scientific Reasons)
5 1 4 2 10 1
7. Why is electoric configuration of 24Cr is 3d 4s instead of 3d 4s while that of Cu is 3d 4s instead of
9 2
3d 4s
Both Cr and Cu shows anomalous electronic configuration in which one electron from higher energy 4s orbital jumps
to the lower energy 3d orbital in order to gain extra stability of being lower energy orbital and attaining single spin
electron (i.e. half-filled orbitals) and opposite spin electrons (fully-filled orbitals) respectively.
5 1 4 2
8. Configuration of 24Cr is 3d 4s instead of 3d 4s .
5 1 4 2
Configuration of chromium is 3d 4s instead of 3d 4s . This anomalous configuration is explained as follows:
(i) 3d and 4s sub-shells are very near to each other, hence electrons of 4s-orbital by jumping may reside in d-orbital.
(ii) Half filled orbitals are more stable (in some cases).
5 1
Thus, in Cr with 3d 4s , both 3d and 4s orbitals attain stability by being half-filled.
10 1 9 2
9. Configuration of 29Cu is 3d 4s instead of 3d 4s .
10 1 9 2
Configuration of copper is 3d 4s instead of 3d 4s . This anomalous configuration is explained as follows:
(i) 3d and 4s sub-shells are very near to each other, hence electrons of 4s-orbital by jumping may reside in d-orbital
(ii) Half filled and completely filled orbitals are more stable.
10 1 10 1
Thus, in Cu with 3d 4s , both 3d and 4s orbitals attain stability by being completely filled (3d ) and half filled (4s )
respectively.
10. Why the elements of group IA are called alkali metals?
The name alkali is derived from the Arabic word means ‘ashes’. These metals are present in the ashes of plants and
gives strong alkaline solution in water. That is why they are called alkali metals.
2Na(s) + 2H2O 2NaOH(aq) + H2(g)
Why the elements of group IIA are called alkaline earth metals?
These metals are widely distributed in the earth crust and give weak alkaline solution in water. That is why they are
called alkaline earth metals.
Ca (s) + 2H2O Ca(OH)2(aq) + H2(g)
11. Group IA elements are called alkali metals while group IIA elements are called alkaline earth metals.
Group IA elements are called Alkali Metals because they yield strong alkalis when their oxides, hydrides or hydroxides
are dissolved in water which are completely soluble in water.
2Na(s) + 2H2O 2NaOH(aq) + H2(g)
Na2O(s) + H2O 2NaOH(aq) +
Group IIA elements are called Alkaline Earth Metals because they themselves and their compounds (CaCO 3) are
found in abundance in the earth crust and yield weak bases when their oxides, hydrides and hydroxides are dissolved
in water which are sparingly soluble in water.
Ca (s) + 2H2O Ca(OH)2(aq) + H2(g)
CaO(s) + H2O Ca(OH)2(aq) +
Conceptual Inorganic Chemistry Text Book -2-
